More rounding! More math!

Once again, I was given 2 numbers, and was told to round them to the nearest whole or tenth. Not really sure what else how to describe it. So there's something in this post, solve this riddle. If you have 20 dollars, and a motorcyclist comes past you and takes 7 and a half dollars from you, how much money do you have left? If you got it right, pay someone 7 and a half dollars to give you a pat in back (Just ask. No need to bribe.). If you thought the answer was 12 and a half, think again. A torn dollar is worthless. ; )